1. Technical Field
Radiation-polymerizable compositions are especially useful as or in a flush vehicle for making flushed pigments. The compositions contain at least one radiation-curable acrylated resin component and a copolymerizable rheology modifier component.
2. Background of Related Art
Pigments are insoluble, fine particle size materials used in a number of applications including ink formulations, coatings, paints and the like to provide color, to hide substrates, to modify the performance properties of coatings and films. In general, most letterpress and lithographic inks are manufactured using dispersed pigment concentrates in a nonaqueous medium.
Pigments are generally produced in an aqueous medium. One of the steps in the manufacture of pigments is precipitation from water. After precipitating the pigment, the pigment/water mixture is filtered to provide a filter cake or presscake containing pigment and moisture. When the pigment is to be incorporated in an organic or non-aqueous system for application to a substrate, it is necessary to remove the residual moisture from the filter cake or presscake. The presscake typically contains 30 to 80% by weight water. Removal of residual water from a presscake is generally accomplished by a process of water displacement known in the art as "flushing."
Generally speaking, flushing is the direct transfer of pigments in an aqueous phase to an oil or nonaqueous phase without intermediate drying. The process for preparing flushed pigments involves the addition of a flushing vehicle, such as an oil or a solution of resins and other additives in an oil or organic solvent, to a presscake. The direct transfer or "flushing" of the pigment particles from the aqueous phase to the oil or non-polar phase is effected by kneading the presscake and vehicle together in a "flusher" or high-shear mixer under heat and vacuum. As the pigment is wet by the flushing vehicle, the water is driven out of the presscake until only a small percentage of water remains. The water that is driven out is removed by standard methods such as decanting. The last traces of water are removed by applying a subatmospheric pressure and/or heat to the presscake. The flushing vehicle in effect "flushes" the water from the pigment resulting in a pigment dispersion in the flushing vehicle. Alkyd resins are sometimes used in addition to the flushing vehicle to assist in the breakout of water from the presscake. Pigments at the presscake stage of manufacture have a particle size which is more suitable for maximum ink gloss and color strength in lithographic ink formulations than pigments in their finished dry form. Accordingly, flushed pigments produced without first drying the pigment are preferred colorants for lithographic ink formulations. When used as a lithographic ink, the flushed pigment normally contains from about 20 to about 70% by weight ink pigment and from about 30 to about 80% by weight flushing vehicle.
A radiation curable grinding vehicle is an admixture of one or more radiation curable oligomers and reactive diluents. The vehicle is used with either a presscake to obtain a UV flush, or with dry pigments on a three roll grinding mill to obtain a lithographic ink.
A flushing vehicle for lithographic ink should exhibit excellent pigment wetting properties when mixed with the highly aqueous presscake during flushing. These pigment wetting properties provide rapid and thorough coverage of the pigment particles and concurrent displacement of water originally bound to or trapped in the particle aggregates. A flushing vehicle with good wetting properties adheres strongly to primary particle units and coats them to provide a steric barrier to reaggregation. The resulting dispersed pigment concentrate exhibits improved color strength, gloss and transparency, as well as reduced bronzing. Representative examples of conventional commercial flushing vehicles used in producing lithographic inks include soluble dibasic acid modified rosin esters, rosin modified phenolic esters, hydrocarbon resins and various specialty alkyds. A flushing vehicle for lithographic inks should generally be stable in storage with respect to viscosity, color and solubility. The prior art compositions used as a flushing vehicle provide inconsistent results. Because of the varying hydrophobic/hydrophillic characteristics of each pigment, it is not unusual to have variability in the time required to remove 100% of the water present in the presscake. For a review of organic pigments and pigment dispersion processes, see U.S. Pat. Nos. 5,420,229 and 5,820,667 and Kirk Othmer's "Concise Encyclopedia of Chemical Technology"; John Wiley & Sons, 1985, pages 869-892, the contents of all three of which are incorporated herein by reference.
The three main technologies being practiced today which make the bulk of the paints, coatings, inks and adhesive industries are solvent borne, water borne and zero volatile organic compounds (VOC). The main film forming process is either drying (evaporation of a solvent from polymer solution) or curing (two or more components reacting to form a thermosetting polymer). While the water borne systems are generally perceived to be more environmentally friendly and acceptable from a waste and pollution standpoint, both solvent and water-based systems are energy intensive, requiring drying ovens to remove the solvent or water. For several years there has been a technological push to eliminate solvents and water, i.e., to develop water less zero VOC systems. Energy curing is one technology which has been investigated with this objective in mind. In an energy curable system, a relatively fluid formulation is instantly converted to a cross-linked polymer when exposed to energy from a visible or ultraviolet (UV) light source or an electron beam (EB). This technology reduces waste and requires less overall energy consumption. In many cases it vastly improves production speeds and produces properties such as high gloss and excellent abrasion resistance. Hence, energy curing is the technology of choice for many applications such as coatings for wood furniture, floor tiles, magazine covers, CD labels and jackets, high gloss optical fibers, electronic encapsulants and stereolithography. Therefore, a radiation-polymerizable composition useful as a flushing vehicle would be very desirable. UV or EB curing can be accomplished by free radical, cationic, anionic, or charge transfer mechanisms.
U.S. Pat. Nos. 3,952,032 and 4,082,710, both of which are incorporated herein by reference, disclose the use of compounds with multiple acrylic radicals as photopolymerizable binders in ultraviolet curable inks and coatings. Other components of the ink composition disclosed in these patents include inert polymers and plasticizers, pigments and inorganic fillers, photoinitiators and various other conventional additives for inks.